1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to an array substrate for an in-plane switching liquid crystal display (IPS-LCD) device and a method of fabricating the array substrate.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The liquid crystal molecules have long, thin, shapes, and have an initial alignment direction including initial pretilt angles. The alignment direction can be controlled by applying an electric field to influence the alignment of the liquid crystal molecules. Due to an optical anisotropy property of liquid crystal, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image having a desired brightness can be produced.
Among the known types of liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superior ability in displaying moving images.
Liquid crystal display (LCD) devices include two substrates spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. In one type of LCD device, each of the substrates includes an electrode with the electrodes of each substrate facing each other. A voltage is applied to each electrode inducing an electric field between the electrodes. The arrangement of the liquid crystal molecules is changed by varying the intensity of the electric field.
Because the electrodes are positioned respectively on each of the two opposing substrates, the electric field induced between the electrodes is perpendicular to the two substrates. Accordingly, LCD devices of this type have a narrow viewing angle because of the vertical electric field. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. An IPS-LCD device includes a pixel electrode and a common electrode on the same substrate.
FIG. 1 is a cross-sectional view illustrating an IPS-LCD device according to the related art. In FIG. 1, an IPS-LCD device 5 includes a first substrate 10 and a second substrate 40 with a liquid crystal layer “LC” interposed therebetween. A pixel region “P” is defined on a first substrate 10. A thin film transistor “T” is formed in the pixel region “P” on the first substrate 10 for use as a switching element. Common electrodes 18 and pixel electrodes 30 are also formed in the pixel region “P.” The thin film transistor “T” includes a gate electrode 14, a semiconductor layer 22, a source electrode 24, and a drain electrode 26. A gate insulating layer 20 is formed between the gate electrode 14 and the semiconductor layer 22. The common electrodes 18 alternate with and are substantially parallel to the pixel electrodes 30 on the first substrate 10. The common electrode 18 is formed of the same material and on the same layer as the gate electrode 14, and the pixel electrode 30 is formed of the same material and on the same layer as the source and drain electrodes 24 and 26. Even though not shown in FIG. 1, a gate line and a data line crossing each other are also formed on the first substrate 10, and a common line, connected to the common electrodes 18, is formed on the first substrate 10.
A second substrate 40 is spaced apart from the first substrate 10. A black matrix 42 is formed on an inner surface of the second substrate 40 facing the first substrate 10. The black matrix 42 on the second substrate 40 corresponds to the thin film transistor “T,” the gate line and the data line on the first substrate 10. A color filter layer 44 including three color filters of red 44a, green 44b, and blue (not shown) is formed on the black matrix 42. The color filter layer 44 corresponds to the pixel region “P” on the first substrate 10. A liquid crystal layer “LC” is interposed between the first substrate 10 and the second substrate 40. The alignment of the liquid crystal layer “LC” is controlled by a horizontal electric field 35 induced between the common electrode 18 and the pixel electrode 30.
FIG. 2 is a plan view illustrating an array substrate for an in-plane switching liquid crystal display (IPS-LCD) device according to the related art. In FIG. 2, a gate line 12 and a data line 28 are formed on a substrate 10. The gate line 12 and the data line 28 cross each other to define a pixel region “P.” A common line 16 is spaced apart from and substantially parallel to the gate line 12. A thin film transistor (TFT) “T” is formed at a crossing portion of the gate line 12 and the data line 28. The TFT “T” includes a gate electrode 14, a semiconductor layer 22 on the gate electrode 14, a source electrode 24 and a drain electrode 26. The gate electrode 14 is connected to the gate line 12 and the source electrode 24 is connected to the data line 28. Common electrodes 18 and pixel electrodes 30, which are substantially parallel to and spaced apart from each other, are formed in the pixel region “P.” The common electrodes 18 contact the common line 16 and extend into the pixel region “P.” The pixel electrodes 30 contact the drain electrode 26 and extend into the pixel region “P.”
The IPS-LCD device has a viewing angle wider than an LCD device using a vertical electric field. However, an increase in the viewing angle of the IPS-LCD device is restricted due to a color inversion along top, bottom, right and left viewing angles. To solve the above problems, an IPS-LCD device having common electrodes and pixel electrodes disposed along a horizontal direction has been suggested.
FIG. 3 is a plan view illustrating an array substrate for an IPS-LCD device according to the related art. In FIG. 3, a gate line 52 and a data line 66 are formed on a substrate 50. The gate line 52 and the data line 66 cross each other to define a pixel region “P.” A thin film transistor (TFT) “T” is connected to the gate line 52 and the data line 66. The TFT “T” includes a gate electrode 54, an active layer 60 over the gate electrode 54, a source electrode 62 on the active layer 60 and a drain electrode 64 spaced apart from the source electrode 62. The gate electrode 54 is connected to the gate line 52, and the source electrode 62 is connected to the data line 66.
In addition, a common electrode 56 and a pixel electrode 72 are formed in the pixel region “P.” The common electrode 56 is formed of the same material and on the same layer as the gate line 52 and the pixel electrode 72 is formed over the common electrode 56 with a gate insulating layer (not shown) and a passivation layer (not shown) between the pixel electrode 72 and the common electrode 56 to prevent contact of the common electrode 56 and the pixel electrode 72. The pixel electrode 72 is formed of a transparent material to improve an aperture ratio. The common electrode 56 includes common horizontal portions 56a, a first common vertical portion 56b and a second common vertical portion 56c. The common horizontal portions 56a are disposed along a horizontal direction, and the first and second common vertical portions 56b and 56c are connected to two end portions of each common horizontal portion 56a, respectively. Further, the pixel electrode 72 also includes pixel horizontal portions 72a, a first pixel vertical portion 72b and a second pixel vertical portion 72c. The pixel horizontal portions 72a are disposed along a horizontal direction, and the first and second pixel vertical portions 72a and 72c are connected to two end portions of each pixel horizontal portion 72a, respectively.
In the IPS-LCD device having the common electrode 56 and the pixel electrode 72 disposed along the horizontal direction, top, bottom, right and left viewing angles are improved by inclining the common electrode 56 and the pixel electrode 72. However, brightness is reduced because the common electrode 56 is formed of an opaque material.